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Homework Help: Planar Kinetics of a Rigid Body Please Help :S

  1. Dec 1, 2005 #1
    Hey, These questions are in my textbook and seeing that I have an exam coming up any help with these problems would be greatly appreciated.

    1) A space shuttle is located in 'deep space', where the effects of gravity can be neglected. It has a mass of 120 Mg, a centre of mass at G, and a radius of gyration kgx = 14 m about the x-axis. It is originally travelling forward at 3km/s (along y-axis) when the pilot turns on the engine at A, creating a thrust T = 600 ( 1 - e^-0.3t) kN, where t is in seconds. Determine the shuttle's angular velocity 2s later.

    The diagram:
    - x-y plane is flat where z points upward
    - the force T is applied 2m above the y-axis
    - the three axis meet @ G
    no other info is given.

    I tried using the principle of linear impulse to find (vg)2 and then using the principle of angular impulse and momentum to find omega ( angular velocity) but I'm not getting the right answer :S

    2) The 2kg rod ACB supports the two 4kg disks at its ends. If both disks are given a clockwise angular velocity (wa)1 = (wb)1 = 5 rad/s while the rod is held stationary and then released, determine the angular velocity of the rod after both disks have stopped spinning relative to the rod due to frictional resistance at the pins A and B. Motion is in the horizontal plane. Neglect friction at pin C.

    From diagram:
    - the radius of the disks at A and B are 0.15m
    - the distance from C to the disks is equal and is 0.75m
    - there is a pin support at pin C

    I tried using conservation of angular momentum... didnt work for some reason :S

    thanks again for any help
  2. jcsd
  3. Dec 1, 2005 #2


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    Science Advisor
    Homework Helper

    1) Have you considered using torque instead?
    This seems like a classical torque problem since you have a constant force & relationship.
    2) Can you list the moment of inertia about C of the final system, and the moment of inertia of each of the disks? A likely error is that you forgot to include the moment of inertia of the disks spinning about C.
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